Hi, tzweck! Welcome to the Seven Forums.
It is important to remember that when you use two monitors, but, then, return to using only one, you disable the extended display option.
Right click your desktop, select "Personalize", then select "Display" (or go to the control panel and select "Display"):
Then select "Change display settings" and change your options:
Alternatively, make sure your bios and/or graphics card is set to Auto-detect your displays monitors.